Coerce model dimension dtypes to numeric

[brynpickering]

Fixes #567

Summary of changes in this pull request:

• Always try to convert data dimensions to numeric and keep any successful attempts.
• Add some tests.
• Add mention of this in the docs.

Reviewer checklist:

• [ ] Test(s) added to cover contribution
• [ ] Documentation updated
• [ ] Changelog updated
• [ ] Coverage maintained or improved

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.86%. Comparing base (872978d) to head (a099c5b). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #575 +/- ## ========================================== + Coverage 95.85% 95.86% +0.01% ========================================== Files 24 24 Lines 3619 3629 +10 Branches 788 724 -64 ========================================== + Hits 3469 3479 +10 Misses 86 86 Partials 64 64 ``` | [Files](https://app.codecov.io/gh/calliope-project/calliope/pull/575?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project) | Coverage Δ | | |---|---|---| | [src/calliope/preprocess/model\_data.py](https://app.codecov.io/gh/calliope-project/calliope/pull/575?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL3ByZXByb2Nlc3MvbW9kZWxfZGF0YS5weQ==) | `100.00% <100.00%> (ø)` | |